1. Field of the Invention
The present invention relates to polymers which exhibit thermothickening properties and the method for forming same. More specifically, the method comprises utilizing an inverse emulsion (i.e, water-in-oil) so that highly concentrated thermothickening polymer emulsions may be formed. The method can involve either the copolymerization of monomers containing multiple alkylene oxide groups or the grafting of alkylene oxide groups onto the chain of a polymer.
2. Technology Description
Reversible thermothickening polymers are known in the art. These polymers represent materials which, when distributed in a solvent, preferably water, increase in viscosity in response to temperature increases but once a certain temperature is reached, the viscosity of the solution will plateau and perhaps ultimately decrease with increasing temperature. The reversible effect comprises that cooling the thickened polymer will cause a reduction in the viscosity of the polymer solution. The goal in using such polymers in industrial applications is to have them exhibit a desired thickening profile at specified temperatures.
These polymers are typically hydrophobically modified water soluble polymers. They are essentially water-soluble (hydrophilic) polymers containing a small portion of hydrophobic groups usually in the form of pendant side chains or terminal end groups. In aqueous media, the hydrophobic groups present in these polymers associate, thus creating the unusual and desired theological properties. Typically these polymers consist of a water soluble polymer backbone modified with polymer grafts that exhibit a low critical solubility temperature (LCST). When subjected to temperature above this LCST the grafts associate and the resulting viscosity increases due these physical crosslinks.
Examples of references which disclose thermothickening polymers and their potential uses include the following documents. Hourdet et al., “Reversible Thermothickening of Aqueous Polymer Solutions”, Polymer, Vol. 35, No. 12, 1994; Sarrazin-Cartalas et al. “Association and Thermal Gelation in Mixtures of Hydrophobically Modified Polyelectrolytes and Nonionic Surfactants, Langmuir, Vol. 10, No. 5, 1994; Loyen et al., “Reversible Thermal Gelation in Polymer/Surfactant Systems”, Langmuir. Vol., 11. No. 4, 1995; Wang et al, “On the Thermal Gelling of Ethyl(hydroxyethyl)cellulose and Sodium Dodecyl Sulfate”, Macromolecules, Vol. 30, No.1, 1997; Sigale et al., “Aspects of Crosslinking Sulfonated Polyacrylamides from Rheological Studies on Their Gels”, CCC 0021-8995/97/061067-06, 1996; WO 97/00275; U.S. Pat. Nos. 4,722,962, 5,681,882 and 5,478,602. To the extent necessary for completion, these documents are expressly incorporated by reference.
In practice, the prior art has suggested the following method for synthesizing the polymers: conventional emulsion polymerization (i.e., oil-in-water emulsion), reverse phase polymerization (used in the production of responsive polymer beads), bulk polymerization, solution polymerization, and micellar polymerization. These synthesis methods are limited in that highly concentrated polymer solutions cannot be easily produced. Accordingly, it would be desirable to develop a synthesis method where high concentrations of thermothickening polymer solids resulting from a polymerization technique can be obtained.
It is further believed that polymers having a molecular weight of greater than 2,000,000 daltons (using a size exclusion chromatograhpy method) cannot be synthesized using the above-mentioned methods. Accordingly, it would desirable to produce polymers having a molecular weight greater than 2,000,000 daltons.